The present invention relates to an imaging system which uses coherent light radiation to expose a layered photosensitive member in an image configuration and, more particularly, to a means and method for suppressing optical interference variations occuring within the photosensitive member which results in a plywooding type of defect in output prints derived from the exposed photosensitive member.
There are numerous applications in the electrophotographic art wherein a coherent beam of radiation, typically from a helium-neon or diode laser, is modulated by an input imaging data signal. The modulated beam is directed (scanned) across the surface of a photosensitive medium such as a photoreceptor drum or belt, or a photosensitive film. Certain classes of photosensitive medium which can be characterized as "layered photoreceptors" have at least a partially transparent layer, referred to as the charge transport layer overlying a charge carrier generation layer and a conductive ground plane. A problem inherent in using these layered photoreceptors, depending upon their physical characteristics, is the creation of two dominant reflections of the incident coherent light on the photoreceptor, e.g., a first reflection from the top surface and a second reflection from the ground plane surface. This condition is shown in FIG. 1 which shows coherent rays 1 and 2 incident on a layered photoreceptor 6 comprising a charge transport layer 7, charge generator layer 8, and a ground plane 9. There are two dominant reflections, one from the top surface of layer 7, and one from the ground plane 9. Depending on the optical path difference as determined by the thickness and index of refraction of layers 7, and 8, rays 1 and 2 can interfere constructively or destructively when they combine to form beam 3. When the additional optical path traveled by ray 1 (dashed rays) is an integer multiple of the wavelength of the light, constructive interference occurs at the top surface, more light exits transport layer 7 and hence less is absorbed within charge generator layer 8 upon subsequent internal reflections. Conversely, a path difference producing destructive interference at the top surface means more light stays in the layer, and more absorption occurs in charge generator layer 8. The difference in absorption in the charge generator layer 8, typically due to layer thickness variations within charge transport layer 7, is equivalent to a spatial variation in exposure on the surface. This spatial exposure variation present in the image formed on the photoreceptor becomes manifest in the output print derived from the exposed photoreceptor. FIG. 2 shows the areas of spatial density variation (at 25.times.) formed on a print when using a photoreceptor of the type shown in FIG. 1 and when illuminated by a He-Ne laser with an output wavelength of 633 nm. A similar variation is present when exposure is by a diode laser. The pattern of light and dark interference fringes look like the grains on a sheet of plywood. Hence the term "plywood effect" is generically applied to this problem.
One method of compensating for the plywood effect known to the prior art is to increase the thickness of, and hence the absorption of, the light by the charge generator layer. For most systems, this leads to unacceptable tradeoffs; for example, for a layered organic photoreceptor, an increase in dark decay characteristics and electrical instability may occur. Another method disclosed in U.S. Pat. No. 4,618,552 is to use a photoconductive imaging member in which the ground plane, or an opaque conductive layer formed above or below the ground plane, is formed with a rough surface morphology to diffusely reflect the light. A third method, disclosed in copending application U.S. Ser. No. 07/523,639, filed on May 15, 1990, now U.S. Pat. No. 5,051,328, and assigned to the same assignee as the present invention, is to form the ground plane of a low reflection material so as to reduce the reflections therefrom.
According to the general principles of the present invention, the plywood effect problem is addressed by using two or more coherent light sources of different wavelengths, and by combining their outputs so as to form a single incoherent scanning beam or two adjacent beams at the photoreceptor. In a first embodiment, a plurality of IR diodes formed as a linear array have their outputs merged into a single scanning beam at an imaging plane. Each of the diodes is operated at a different wavelength to obtain an essentially incoherent beam. In a second aspect of the invention, two optimum output wavelengths are prescribed so that absorptance maxima of one and minima of the other coincide. In a still further extension of the present invention, a linear array of diodes of differing wavelengths are sequentially addressed to achieve a uniform integrated absorption distribution at the photoreceptor surface.
More particularly, the present invention relates to a light scanning system comprising:
a photosensitive image recording member comprising at least a transparent photoconductive charge transport layer overlying a charge generator layer and a conductive ground plane,
a diode laser array including a plurality of diodes, each diode operating at a different output wavelength,
means for supplying video signal information to the diodes to provide for emission of a plurality of light beams from the array with each of the light beams modulated in accordance with the video signal information and at the characteristic wavelength,
optical means for focusing the plurality of light beams to form a merged beam, or adjacent beams, at the surface of the recording member, and
scanning means for scanning the light beam across the linear portion of the image recording member surface, wherein the merged multi-wavelength scan beam is almost uniformly absorbed within the layers.
While it is known in the art to combine laser beam outputs from multiple laser sources to achieve higher power outputs (see U.S. Pat. No. 4,725,131), it has not previously been known to combine multiple laser sources with different wavelengths for the purpose of eliminating plywooding effects in a layered photosensitive image surface.